Elevator systems are widely used for transporting people or goods from one point to another. An elevator system typically includes an elevator car connected to a counterweight by way of hoisting ropes, such as, steel cables and adapted to move vertically in an elevator hoistway or elevator shaft. These hoisting ropes extend over a sheave or machine located in a machine room above the elevator car. A motor connected to the machine provides power to move the elevator car between two positions. The machine (and specifically the motor of the machine) is provided with a brake system of one or more brakes to stop the elevator car as desired and to prevent undesired motion of the elevator car.
In addition to the brake system, most elevator systems are equipped with safety systems for initiating an emergency stop of the elevator car in case of any over speed situations of the elevator car. One such emergency safety feature is a speed governor, which typically includes a governor rope passing over the speed governor pulley and extending downward to a tensioning pulley located at the bottom of the elevator shaft. The speed governor is adapted to detect an over speed situation of the elevator car based upon a ratio of the rotational velocity of the governor pulley proportional to the speed of the elevator car. The governor pulley is connected to a centrifugally operated trip device, which engages a mechanical device that will activate the safeties of the elevator car when the elevator car reaches a predetermined over speed condition.
The speed governor may either be located within the machine room, hoistway or, on top of the elevator car. In operation, as the elevator car travels up and down the elevator shaft, flyweights provided on the governor pulley move outwardly due to the centrifugal force imparted thereon by the rotating governor pulley. When the speed of the elevator car exceeds a rated speed by a predetermined value, the flyweights are driven outwardly and are caused to trip an over speed switch which cuts off (or substantially reduces) power to the drive motor and simultaneously sets the brake. In the event the elevator car speed continues to increase, the further outward motion of the flyweights cause the flyweights to contact and activate a mechanical device, and engage safeties 5 provided on the elevator car to arrest motion of the elevator car.
As the normal rated speeds of modern elevator systems have continued to rise, so has the need to have more robust and reliable safety systems in elevator cars. Safety code requirements for elevators and escalators require that for elevator systems operating at certain speeds, such as, at or over two hundred feet per minute (200 FPM or 1.016 meters/second) and having a speed governor with an over speed switch that operates at the same over speed tripping point of the mechanical device of the speed governor, a speed reducing switch be employed for reducing speed of an elevator car in over speed conditions of the elevator car in addition to the over speed switch of the speed governor. These code requirements further require that the speed reducing switch be of a manually reset type such that after tripping (e.g., activating) the speed reducing switch, the elevator car remains in an inoperative state until the switch is manually reset. The code additionally states that when the speed reducing switch is provided, its speed trip point be about ten percent (10%) below the speed governor over speed switch.
Accordingly, it would be beneficial if an electronic speed reducing switch in compliance with the code requirements were developed.